In the rambling letter, postmarked Friday, April 3, -- the day of the massacre -- and mailed to Syracuse's News 10 Now, the author allegedly identifies himself and admits to the shootings: "I am Jiverly Wong shooting the people."

Handwritten in all capital letters, the letter conveys a sense of isolation, prejudice and persecution, saying he couldn't accept his "poor life" and will take "at least two people with me go to return to the dust of earth."

Wong, an immigrant from Vietnam who reportedly attended language classes at the civic association where police say he opened fire, apologizes for his weak English skills in the letter. Through somewhat broken English, the letter explains the motives behind the shooting, pointing to a history of exploitation and ridicule by police officers.

"Of course you need to know why I shooting? Because undercover cop gave me a lot of a** during eighteen years," he wrote.

The letter says that police officers from California and New York spread rumors about him, stole money from him, tried to crash his car 32 times and even watched and touched him during his sleep.

"Cop wait until midnight when I off the light and went to the bed. Cop unlock my door and came in take a sit in my room ... on the thirteen time had three time touch me when I sleeping."

Wong also reportedly expressed annoyance about losing his job at a vacuum manufacturer.

"Right now I still get unemploment benefit of the company Shop Vac Endicott. New York State Department of Labor was cheat and unpaid from December 1st 2008 to December 28th 2008 I already claim weekly benefit from that date," it reads.

Officials had still not indentified a motive for the shootings, and it was initially believed Wong was discharged from IBM.

Along with the letter came photos of Wong with guns, as well as a gun permit and driver's license, according to the Associated Press. Local police say they will attempt to verify the authenticity of the letter and consider it as evidence in the investigation. They added that mental health professionals will analyze the letter to learn more about the psychology of the murderer.

Binghamton Rampage Leaves 14 Dead

As many as 14 people were shot dead in the murderous three-minute shooting rampage Friday inside an upstate New York civic association building that caters to immigrants, according to federal and state authorities. Police sources identified the gunman as Jiverly Wong.

According to Binghamton Police Chief Joe Zikuski, the gunman entered the one-story American Civic Association in downtown Binghamton at 10:31 a.m. Friday and began his shooting spree.

By 10:33 a.m., the shooting was over and 14 people -- including the gunman -- lay dead, the chief said.

At least four people were listed in critical condition. Earlier, sources said as many as 26 people were wounded.

Police were interviewing Wong's family members and executing a search warrant at his home in an effort to establish a motive. Officials were seen removing a gun rifle case, two hard drives, two briefcases and two brown paper bags from the home.

Police Chief: 'He Made Sure Nobody Could Escape'

Officials said Wong entered the civic association armed with two pistols -- a 9 mm and a .45 caliber handgun. A satchel around the gunman's neck carried high capacity magazines, a survival knife and a flashlight, according to police.

Wong backed his car up to the rear door of the building.

"Obviously, it was premeditated. He made sure nobody could escape," Zikuski said.

Wong then headed to the front where he allegedly began executing people.

Wong, 41, also known as Linh Phat Wong, was from Johnson City, N.Y.

He allegedly burst into the civic center wearing a bright green nylon jacket and dark-rimmed glasses and promptly shot two female receptionists.

One woman died, but the second woman survived.

"She pretended she was dead," Zikuski said. As the gunman headed for a room off the reception area, "she crawled underneath the desk and sometime after that called us," he said.

Most of the people killed or injured were in one classroom taking a citizenship exam.

The police chief said 37 people were safely removed from the building, 26 of whom barricaded themselves in the building's boiler room.

Police arrived just two minutes after the receptionist called 911. Though the shooting lasted only minutes, it took police three hours to make sure the shooter wasn't still alive and laying in wait for more victims.

Zhanar Tokhtabayeva, a 30-year-old from Kazakhstan, told the Associated Press she was in an English class when she heard a shot and her teacher screamed for everyone to go to the storage room.

"I heard the shots, every shot. I heard no screams, just silence, shooting," she said. "I heard shooting, very long time ... and I was thinking, when will this stop? I was thinking that my life was finished."

Sister Stunned at News of Shooting

When the carnage was over, Wong's body was found on the first floor with a hunting knife jammed into the waistband of his pants.

"He shot those people? No. No," said a woman who identified herself as Wong's sister, but would not give her name when reached by ABC News.com.

She said her brother went to take classes today at the civic association and that she had not heard from him since. She said she did not know that he was involved in the shooting.

"I'm going to pass out," she said, and she hung up the phone.

A neighbor who lived on the same block as Wong and his family described the family as "quiet" and said they mostly kept to themselves.

"They were nice people," said the neighbor who identified herself only as Darlene. "They were good neighbors."

Police also contacted Wong's sister to say her brother was dead. She told police that Wong was attending language classes at the civic center. She said Wong is a U.S. citizen and has been in this country for 28 years.

U.S. Rep. Maurice Hinchey, a Democrat whose district includes Binghamton, initially told the Associated Press Wong was recently laid off from IBM in Johnson City -- but sources later told ABC News that it appeared that the Wong who worked at IBM was Henry Voong, "an older gentleman" who was believed to be the suspect's father, and that he had not been laid off.

Fred McNeese, an IBM spokesperson, said that because the police have not officially identified the shooter, the company is "unable to confirm any connection to IBM" at this time and has no other comment.

The letter allegedly written by Wong points to his employment at a vacuum manufacturing company, Shop Vac Endicott.

Several hours after the shooting began, hostages started exiting the building and police declared the gunman dead.

By 1:30 p.m., police began escorting people out of the building, but continued to treat the building as a crime scene where the shooter might still be lurking.

Eyewitness Account

The American Civic Association released a statement early Saturday morning.

"We are stricken with grief about today's horrific assault and share this grief with the victims' families, our community and the entire nation."

Eyewitness Nick Masucci, a community college student and Binghamton resident, said he watched as many as 20 people exit the building and get patted down by police.

"They look like immigrants, lots of different cultures coming out of there. Some people are getting patted down. The police are taking a lot of precautions, they're still taking cover," he said.

Several people were removed from the building on stretchers. Others left with their hands on their heads and were searched by police.

Earlier Friday, police called Broome Community College to find someone who could speak fluent Vietnamese to "assist police with translation," said college spokesman Richard David.

Obama Comments on Shooting

People were told in nearby buildings to stay away from windows and Binghamton High School is under a lockdown as police use the school as a staging area. The school is a block and a half away from the shooting site.

President Obama issued a statement from the NATO summit he was attending in France.

"Michelle and I were shocked and deeply saddened to learn about the act of senseless violence in Binghamton, N.Y., today. Our thoughts and prayers go out to the victims, their families and the people of Binghamton," he said.

New York Gov. David Paterson called the shooting "the worst tragedy and senseless crime in the history of the city."

"We all just have profound sorrow and sadness," he said.

Additonal reporting by ABC News' Ned Potter and Ki Mae Heussner, as did the Associated Press.